U.S. Pat. No. 3,099,859 discloses a so-called triplex system for producing tread strips for motor vehicle tyres, the tread strip being formed from three different rubber mixtures. The three different mixtures at a time are extruded simultaneously into a tread strip injection moulding head and are discharged therefrom through a nozzle to form a tread strip. The three mixtures have very different properties from one another. Thus, for example, a highly wear-resistant rubber mixture is needed to form the tread surface, a very resilient rubber mixture is needed to produce the lateral surfaces of the tread strip and a very durable rubber mixture is needed to form the base of the strip.
Large quantities of each individual mixture must be prepared for the mass-production of automotive tyres, such individual mixtures being prepared in batches in internal mixers and rolling-mills. Lengths of the mixture are then removed from the rolling-mill and deposited, in a zig-zag manner, on wooden pallets or the like. In other words, a ready-mixed batch of material is deposited on one pallet at a time.
Each batch is produced using accurately-weighed amounts of the constituents of the mixture in both the internal mixers and in the rolling-mills. However, the mixing and blending of the constituents may vary in the internal mixers and rolling-mills with the result that the quality and/or characteristics may vary from one batch to another.
During further processing in a triplex system, such variations can cause, for example, the properties of the tread surface of an extruded tread strip to vary from batch to batch, which is obviously undesirable.
Moreover, it is evident that a triplex system comprising three separate extruders each requiring appropriate gears and drive motors, is very large, occupies a considerable amount of space and is both difficult and expensive to operate.